Download Lecture 11: Reproduction III

BIO 2, Lecture 11
REPRODUCTION III:
HEREDITY, MENDEL’S LAWS, AND
NON-MENDELIAN INHERITANCE
• What genetic principles govern the
passing of traits from parents to
offspring?
• The “blending” hypothesis is the idea that
genetic material from the two parents
blends together (like blue and yellow paint
blend to make green)
• The “particulate” hypothesis is the idea
that parents pass on discrete heritable
units (genes)
• Gregor Mendel (1822-1884) was the
first to demonstrate that genetic traits
are inherited in a particulate fashion.
• Although chromosomes and genes had
not yet been discovered, Mendel
uncovered the two basic principles
governing their heredity by breeding
garden peas in carefully planned
experiments
• These two basic principles became
known as Mendel’s Laws
• Mendel Laws do nothing more than
describe the behavior of chromosomes
(and the genes they carry) during
meiosis.
• If you understand meiosis, you can easily
understand Mendel’s Laws
• Advantages of pea plants for genetic
study:
– There are many varieties with distinct
heritable features (such as flower color)
– Each feature has alternative traits (such as
purple or white flowers)
– Mating of plants can be controlled
– Each pea plant has sperm-producing organs
(stamens) and egg-producing organs (carpels)
– Cross-pollination (fertilization between
different plants) can be achieved by dusting
one plant with pollen from another
TECHNIQUE
1
2
Parental (P)
generation
Stamens
Carpel
3
4
RESULTS
5
First filial (F1)
generation offspring
• Mendel chose to track only those
characters that varied in an either-or
manner (e.g. either purple or white
flowers)
• He also used varieties that were truebreeding (plants that produce offspring
of the same variety when they selfpollinate)
• In a typical experiment, Mendel mated
two contrasting, true-breeding varieties,
a process called hybridization
• The true-breeding parents are the P
generation
• The hybrid offspring of the P generation
are called the F1 generation
• When F1 individuals self-pollinate, the F2
generation is produced
• When Mendel crossed true-breeding
white and purple flowered pea plants, all
of the F1 hybrids were purple
• When Mendel crossed the F1 hybrids,
many of the F2 plants had purple flowers,
but some had white
• Mendel discovered a ratio of about
three to one purple to white flowers in
the F2 generation
EXPERIMENT
P Generation
(true-breeding
parents)
F1 Generation
(hybrids)

Purple
flowers
White
flowers
All plants had
purple flowers
F2 Generation
705 purple-flowered 224 white-flowered
plants
plants
• Mendel called the purple flower color a
dominant trait and the white flower
color a recessive trait
• Mendel observed the same pattern of
inheritance in six other pea plant
characters, each represented by two
traits
• What Mendel called a “heritable factor”
is what we now call a gene
• What Mendel called a “trait” is what we
now call an allele
GENE
Dominant
Allele
Recessive
Allele
• Mendel developed a hypothesis to explain
the 3:1 inheritance pattern he observed
in F2 offspring
• Four related concepts make up this model
• These concepts can be directly related to
the behavior of chromosomes and the
genes they carry during meiosis
• The first concept is that alternative
versions of genes account for variations
in inherited characters
• For example, the gene for flower color in
pea plants exists in two versions, one for
purple flowers and the other for white
flowers
• These alternative versions of a gene are
now called alleles
• Each gene resides at a specific locus on a
specific chromosome
Allele for purple flowers (P)
Locus for flower-color gene
Homologous
pair of
chromosomes
(not sister
chromatids!)
Allele for white flowers (p)
• The second concept is that for each
character an organism inherits two
alleles, one from each parent
• The two alleles at a locus on a
chromosome may be identical, as in the
true-breeding plants of Mendel’s P
generation (PP) or (pp)
• Alternatively, the two alleles at a locus
may differ, as in the F1 hybrids (Pp)
• The third concept is that if the two
alleles at a locus differ, then one (the
dominant allele) determines the
organism’s appearance, and the other
(the recessive allele) has no noticeable
effect on appearance
• In the flower-color example, the F1
plants had purple flowers because the
allele for that trait is dominant
• The fourth concept, now known as
Mendel’s First Law, or the Law of
Segregation, states that the two alleles
for a heritable character separate
(segregate) during gamete formation and
end up in different gametes
• Thus, an egg or a sperm gets only one of the
two alleles that are present in the somatic
cells of an organism
• This segregation of alleles corresponds to
the distribution of homologous
chromosomes to different gametes in
meiosis (i.e. each gamete is haploid)
• Mendel’s segregation model accounts for
the 3:1 ratio he observed in the F2
generation of his numerous crosses
• The possible combinations of sperm and
egg can be shown using a Punnett square,
a diagram for predicting the results of a
genetic cross between individuals of
known genetic makeup
• A capital letter represents a dominant
allele, and a lowercase letter represents a
recessive allele
P Generation
Purple flowers White flowers
Appearance:
Genetic makeup:
PP
pp
Gametes:
P
p
P Generation
Purple flowers White flowers
Appearance:
Genetic makeup:
PP
pp
Gametes:
p
P
F1 Generation
Appearance:
Genetic makeup:
Gametes:
Purple flowers
Pp
1/
2
P
1/
2
p
P Generation
Purple flowers White flowers
Appearance:
Genetic makeup:
PP
pp
Gametes:
p
P
F1 Generation
Appearance:
Genetic makeup:
Gametes:
Purple flowers
Pp
1/
2
1/
2
P
Sperm
F2 Generation
P
p
PP
Pp
Pp
pp
P
Eggs
p
3
1
p
• An organism with two identical alleles for
a character is said to be homozygous for
the gene controlling that character
• An organism that has two different
alleles for a gene is said to be
heterozygous for the gene controlling
that character
• Unlike homozygotes, heterozygotes are
not true-breeding
• Because of the different effects of
dominant and recessive alleles, an organism’s
traits do not always reveal its genetic
composition
• Therefore, we distinguish between an
organism’s phenotype, or physical
appearance, and its genotype, or genetic
makeup
• In the example of flower color in pea plants,
PP and Pp plants have the same phenotype
(purple) but different genotypes
3
Phenotype
Genotype
Purple
PP
(homozygous)
Purple
Pp
(heterozygous)
1
2
1
Purple
Pp
(heterozygous)
White
pp
(homozygous)
Ratio 3:1
Ratio 1:2:1
1
• How can we tell the genotype of an
individual with the dominant phenotype?
• Such an individual must have one dominant
allele, but the individual could be either
homozygous dominant or heterozygous
• The answer is to carry out a testcross:
breeding the mystery individual with a
homozygous recessive individual
• If any offspring display the recessive
phenotype, the mystery parent must be
heterozygous
TECHNIQUE

Dominant phenotype, Recessive phenotype,
unknown genotype:
known genotype:
PP or Pp?
pp
Predictions
If PP
Sperm
p
p
P
Pp
Eggs
If Pp
Sperm
p
p
or
P
Pp
Eggs
P
Pp
Pp
pp
pp
p
Pp
Pp
RESULTS
or
All offspring purple
1/2
offspring purple and
offspring white
1/2
• Mendel derived the law of segregation by
following a single character
• The F1 offspring produced in this cross
were monohybrids, individuals that are
heterozygous for one character
• A cross between such heterozygotes is
called a monohybrid cross
• Mendel identified his second law of
inheritance by following two characters
through a cross at the same time
• Crossing two true-breeding parents
differing in two characters produces
dihybrids in the F1 generation,
heterozygous for both characters
• A dihybrid cross, a cross between F1
dihybrids, can determine whether two
characters are transmitted to offspring
as a package or independently
EXPERIMENT
P Generation
YYRR
yyrr
Gametes YR

F1 Generation
YyRr
Hypothesis of
dependent
assortment
Predictions
Hypothesis of
independent
assortment
Sperm
or
Predicted
offspring of
F2 generation
1/
2
yr
Sperm
1/ YR 1/
2
2 yr
1/
4
1/
4
Yr
1/
4
yR 1/4 yr
YR
YYRR YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
YR
YYRR
Eggs
1/
2
1/
4
YyRr
Yr
Eggs
yr
YyRr
3/
4
1/
4
yyrr
yR
1/
4
Phenotypic ratio 3:1
1/
4
yr
9/
16
108
3/
16
3/
16
1/
16
Phenotypic ratio 9:3:3:1
RESULTS
315
YR
1/
4
101
32
Phenotypic ratio approximately 9:3:3:1
• Using a dihybrid cross, Mendel developed
the his Second Law, the Law of
Independent Assortment
• The Law of Independent Assortment states
that each pair of alleles segregates
independently of each other pair of alleles
during gamete formation
• Strictly speaking, this law applies only to
genes on different, non-homologous
chromosomes
• Genes located near each other on the same
chromosome tend to be inherited together
• Mendel’s Law of Independent Assortment
reflects the behavior of homologous pairs
of chromosomes during meiosis
• Homologous pairs line up at the
metaphase plate independently of all
other homologous pairs
• Therefore, if a diploid cell is heterozygous
for two genes located on different
chromosomes, the line-up could place both
dominant alleles on the same size of the
plate or on opposite sides of the plate
Possibility 1
a
a
b
A
A
B
B
b
Possibility 2
a
a
A
A
Two equally probable
arrangements of B
chromosomes at
B
metaphase I
a
a
A
b
b
A
a
a
B
B
B
B
b
b
A
A
b
b
Metaphase II
a
a
A
A
a
a
A
A
b
b
B
B
B
B
b
b
Combination 1 Combination 2
Daughter
cells
Combination 3 Combination 4
• Mendel’s laws of segregation and
independent assortment reflect the rules
of probability
• When tossing a coin, the outcome of one
toss has no impact on the outcome of the
next toss
• In the same way, the alleles of one gene
segregate into gametes independently of
another gene’s alleles
• The multiplication rule states that the
probability that two or more
independent events will occur together
is the product of their individual
probabilities
• Probability in an F1 monohybrid cross can be
determined using the multiplication rule
• Segregation in a heterozygous plant is like
flipping a coin: Each gamete has a chance of
carrying the dominant allele and a chance of
carrying the recessive allele

Rr
Segregation of
alleles into eggs
Rr
Segregation of
alleles into sperm
Sperm
1/
2
1/
2
R
R
R
Eggs
1/
2
r
R
R
r
1/
4
1/
4
r
r
1/
2
r
R
r
1/
4
1/
4
• The rule of addition states that the
probability that any one of two or more
exclusive events will occur is calculated
by adding together their individual
probabilities
• The rule of addition can be used to figure
out the probability that an F2 plant from
a monohybrid cross will be heterozygous
rather than homozygous
• We can apply the multiplication and
addition rules to predict the outcome of
crosses involving multiple characters
• A dihybrid or other multi-character
cross is equivalent to two or more
independent monohybrid crosses
occurring simultaneously
• In calculating the chances for various
genotypes, each character is considered
separately, and then the individual
probabilities are multiplied together
• The relationship between genotype and
phenotype is rarely as simple as in the pea
plant characters Mendel studied
• Many heritable characters are not
determined by only one gene with two
alleles
• However, the basic principles of
segregation and independent assortment
apply even to more complex patterns of
inheritance
• Inheritance of characters by a single
gene may deviate from simple Mendelian
patterns in the following situations:
– When alleles are not completely dominant or
recessive
– When a gene has more than two alleles
– When a gene produces multiple phenotypes
• Complete dominance occurs when
phenotypes of the heterozygote and
dominant homozygote are identical
• In incomplete dominance, the phenotype
of F1 hybrids is somewhere between the
phenotypes of the two parental varieties
• In codominance, two dominant alleles
affect the phenotype in separate,
distinguishable ways
P Generation
Red
CRCR
Complete dominance,
incomplete dominance,
or codominance?
White
CWCW
Gametes
CR
CW
Pink
CRCW
F1 Generation
Gametes 1/2 CR
1/
2
CW
Sperm
1/
2
CR
1/
2
CW
F2 Generation
1/
2
CR
Eggs
1/
2
CRCR
CRCW
CRCW
CWCW
CW
• Most genes exist in populations in more
than two allelic forms (multiple alleles)
• For example, the four phenotypes of the
ABO blood group in humans are determined
by three alleles for the enzyme (I) that
attaches A or B carbohydrates to red blood
cells: IA, IB, and i.
• The enzyme encoded by the IA allele adds
the A carbohydrate, whereas the enzyme
encoded by the IB allele adds the B
carbohydrate; the enzyme encoded by the i
allele adds neither
Allele
IA
IB
Carbohydrate
A
B
none
i
(a) The three alleles for the ABO blood groups
and their associated carbohydrates
Genotype
Red blood cell
appearance
Phenotype
(blood group)
IAIA or IA i
A
IBIB or IB i
B
IAIB
AB
ii
O
(b) Blood group genotypes and phenotypes
• Most genes have multiple phenotypic
effects, a property called pleiotropy
• For example, pleiotropic alleles are
responsible for the multiple symptoms of
certain hereditary diseases, such as cystic
fibrosis and sickle-cell disease
• In addition, some traits may be determined
by two or more genes (multigenic traits)
• In epistasis, a gene at one locus alters
the phenotypic expression of a gene at a
second locus
• For example, in mice and many other
mammals, coat color depends on two genes
• One gene determines the pigment color (with
alleles B for black and b for brown)
• The other gene (with alleles C for color and c
for no color) determines whether the
pigment will be deposited in the hair
BbCc
Sperm
1/
Eggs
1/
1/
1/
1/
4 BC
1/

4 bC
BbCc
1/
4 Bc
1/
4 bc
4BC
BBCC
BbCC
BBCc
BbCc
BbCC
bbCC
BbCc
bbCc
BBCc
BbCc
BBcc
Bbcc
BbCc
bbCc
Bbcc
bbcc
4 bC
4 Bc
4 bc
9
: 3
: 4
• The more genes control a trait, the more
complicated the inheritance becomes
• Skin color in humans is an example of a
trait that is under the control of many
genes (at least 8)
• This type of inheritance usually results in
a bell curve distribution of phenotypes in
the population

AaBbCc
Sperm
1/
Eggs
1/
8
1/
8
1/
8
1/
8
1/
1/
8
1/
1/
8
8
1/
8
1/
64
15/
8
1/
1/
8
8
8
1/
8
1/
8
1/
8
AaBbCc
8
1/
Phenotypes:
64
Number of
dark-skin alleles: 0
6/
1
64
15/
64
2
20/
3
64
4
6/
64
5
1/
64
6
• Another departure from Mendelian
genetics arises when the phenotype for a
character depends on environment as well
as genotype
• The norm of reaction is the phenotypic
range of a genotype influenced by the
environment
• For example, hydrangea flowers of the same
genotype range from blue-violet to pink,
depending on soil acidity
• Norms of reaction are generally broadest
for polygenic characters
• Such characters are called multifactorial
because genetic and environmental
factors collectively influence phenotype
• For multifactorial traits, an organism’s
phenotype reflects its overall genotype
and unique environmental history
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• Humans are not good subjects for genetic
research
– Generation time is too long
–
–
Parents produce relatively few offspring
Breeding experiments are unacceptable
• However, basic Mendelian genetics endures
as the foundation of human genetics, just as
for other eukaryotic organisms
• Pedigrees can be used to track traits
through families and make predictions about
future offspring
• We can use the multiplication and addition
rules to predict the probability of specific
phenotypes
Key
Male
Female
Affected
male
Affected
female
Mating
Offspring, in
birth order
(first-born on left)
1st generation
(grandparents)
2nd generation
(parents, aunts,
and uncles)
Ww
ww
ww
Ww ww ww Ww
Ww
Ww
ww
3rd generation
(two sisters)
WW
or
Ww
Widow’s peak
ww
No widow’s peak
(a) Is a widow’s peak a dominant or recessive trait?
Parents
Normal
Aa

Normal
Aa
Sperm
A
a
A
AA
Normal
Aa
Normal
(carrier)
a
Aa
Normal
(carrier)
aa
Albino
Eggs
• If a recessive allele that causes a disease
is rare, then the chance of two carriers
meeting and mating is low
• Consanguineous matings (i.e., matings
between close relatives) increase the
chance of mating between two carriers of
the same rare allele
• Most societies and cultures have laws or
taboos against marriages between close
relatives
• Cystic fibrosis is the most common lethal
genetic disease in the United
States,striking one out of every 2,500
people of European descent
• The cystic fibrosis allele results in
defective or absent chloride transport
channels in plasma membranes
• Symptoms include mucus buildup in some
internal organs and abnormal absorption
of nutrients in the small intestine
• Sickle-cell disease affects one out of
400 African-Americans
• The disease is caused by the substitution
of a single amino acid in the hemoglobin
protein in red blood cells
• Symptoms include physical weakness, pain,
organ damage, and even paralysis
• Some human disorders are caused by
dominant alleles
• Dominant alleles that cause a lethal
disease are rare and arise by mutation
• Achondroplasia is a form of dwarfism
caused by a rare dominant allele
Parents
Dwarf
Dd

Normal
dd
Sperm
D
d
d
Dd
Dwarf
dd
Normal
d
Dd
Dwarf
dd
Eggs
Normal